Investigation into Replacement of Viton ‘O’-Rings

Abstract

— Fluorocarbon ‘O’-rings to DTD5612A:1982 have been used for many years for sealing at −40°C. The UK Department for Transport (DfT) issued a letter on 12 March 2001 raising concerns over the suitability of these ‘O’-rings for service temperatures down to −40°C as required by the regulations for certain types of packages containing radioactive material, and requested that the technical justification for continued use was reviewed. Gravatom Engineering Systems Ltd performed a low-temperature test, at Harwell, using design no 3406 with new fluorocarbon ‘O’-rings and bolts. The test proved that these ‘O’-rings are not suitable for temperatures down to −40°C since the seals would not hold a vacuum at temperatures lower than −24°C. This necessitated an investigation to find a suitable ‘O’-ring material with a leak rate pelformance of 10⁻⁵ Pa.m³. s⁻¹ SLR at −40°C to +330°C since this represents the widest temperature range of the AWE Type B(U) containers. Various seal manufacturers were contacted, and following further investigation two seal materials presented themselves as possible options: Viton GLT, a fluorocarbon manufactured by Parker-USA (Parker compound V0835–75) with CEETAK Ltd acting as agents in the UK, and EPDM-30H developed by BNFL and manufactured by James Walker & Co Ltd. Following testing, the EPDM-30H seals proved to be the most suitable sealing material option for an operating temperature range of −40°C to +330°C. The seal achieved a leak rate at −40°C of better than 8.0 × 10⁻¹⁰ Pa.m³.s⁻¹ which is well within the equivalent target leak rate of less than 1.2 × 10⁻⁴ Pa. m³.s⁻¹. The seals also remained effective down to at least −50°C with no loss in sealing pelformance. A leak rate of better than 7.2 × 10⁻⁶ Pa.m³.s⁻¹. was achieved at +330°C which is within the target leak rate of less than 6.4 × 10m⁻⁵ Pa.mm³.s⁻¹. The interspace pressure increased slightly during heating, but recovered fully on cooling to give a leak rate at ambient temperature of 1.0 × 10⁻¹⁰ Pa.mm³.s⁻¹. The EPDM-30H seals proved satisfactory at high temperature, with excellent recovery on cooling. It was concluded that EPDM-30H seals may be used as a direct replacement for Viton where the temperature range does not exceed −40°C to +330°C and where the higher temperature of that range is of relatively short duration (i.e. less than 24 h).